This invention relates to drilling fluids used in the drilling, completion and workover of subterranean oil and gas wells, and more particularly, relates to a non-polluting additive for water-base drilling fluids to prevent drill pipe or casing sticking, when drilling or running casing, especially through depleted or subnormal pressure formation zones.
The most common form of drilling a subterranean oil or gas well is a rotary system in which a column of drill pipe is rotated to turn a multi-pronged drilling bit attached to the bottom of the pipe. The drill bit cuts into the earth formations, "making hole" and producing cuttings ("drilled solids") which must be removed as drilling continues. To do this, a drilling fluid is used. A liquid drilling fluid is a suspension in which fine particles of solid materials are mixed throughout the liquid, and is called drilling mud. In water-base muds, water is the liquid and is said to be the continuous phase; the particles are the dispersed phase. The dispersed phase of a water mud may include emulsified oil, and such muds are called oil emulsion muds. An oil-base mud has an oil continuous phase. Oxidized ("blown") asphalt particles may be mixed in oil base muds or oil emulsion muds to provide viscosity and/or wall building properties.
To remove cuttings, a drilling mud is circulated down the drill pipe, exits at the bit, and heads back up the bore hole in the annulus between the outside of the drill string and the wall of the bore hole. This circulation of the drilling mud removes cuttings made by the bit from the bottom of the hole and lifts them to the surface for disposal.
It is desirable for the drilling fluid to have a low viscosity when circulating down the drill pipe and out the bit, and to have a higher viscosity in the annulus. Low viscosity is important in removal of cuttings (chips). Unless cuttings are removed from beneath the bit before the next bit tooth arrives, the cuttings are reground. This decreases drilling penetration rates, which can increase costs of the well. Once in the annulus, the cuttings, which are generally heavier than the drilling mud, tend to settle downward. The upward velocity of the fluid in the annulus must be higher than the settling rate to bring the cuttings to the top of the hole. A higher viscosity in the annulus than at the bit reduces the settling rate.
A primary function of a drilling mud is to prevent formation fluids from flowing into the wellbore. This is usually done by adding a commercial weighting material, like barite, to the fluid to increase the density. When the hydrostatic pressure in the wellbore is higher than the pressure in a permeable formation, the mud tries to flow into the formation. A permeable formation operates as a strainer, holding back the solids in the mud while permitting the water to pass into pore spaces. As a result, a layer of solids called a filter cake is deposited on the exposed formation. Excessive water loss into a permeable formation has several undesirable effects, one of which is that the filter cake build up can cause tight places in the hole and drill string sticking.
No cake is formed on impermeable shale, but water from the drilling mud wets the surface and may make it cave in or slough before casing can be set. Blown petroleum asphalt treated to be water dispersible has been used in conventional water base muds where an asphaltic product is desirable for stabilization of troublesome shales. The asphalt particles are believed to plug microfractures and pore spaces in the shale, helping seal the shale from influx of water.
One of the most noticeable effects of filter cake build up in the mud is differentially stuck drill pipe. Differential sticking is a problem with any deviated hole, and is particularly likely to occur when drilling high angle holes from offshore platforms. It happens when a portion of the drill string lies against a side of a deviated hole where a filter cake has built up adjacent a permeable formation. While the pipe is being rotated, it is lubricated by a film of mud, and the hydrostatic pressure exerted by the drilling fluid is equal on all sides of the pipe. However, when rotation of the pipe is stopped, the portion of the pipe in contact with the filter cake is isolated from the mud column, and the differential of the hydrostatic pressure of the mud and the formation pressure exerted on opposing sides of the pipe presses the pipe into the filter cake and causes drag when an attempt is made to pull the pipe. This force (F) required to pull free is the product of the coefficient of friction (u) and the force pushing the drill string against the formation. The pushing force is the product of the area (A) of contact between the drill pipe and the cake, and the excessive hydrostatic pressure (P) at the contact area, which is equal to the difference between the pressure of the mud in the hole and the formation pressure. Thus, the force to pull free is: EQU F=uAP
The coefficient of friction u between the steel drill pipe and the mud cake depends on the composition of the mud. The area of wall contact A depends not only on the diameters of the pipe and the borehole, but also on filter cake thickness. The force to pull free may exceed the power of surface equipment, and unless the stuck pipe can be released, the hole, the drill string and equipment on the drill string below where the pipe is stuck may be lost. This can cause expensive remedial measures and usually side track (directional) drilling above the stuck pipe if that much of the hole is not to be abandoned.
The problem of differential sticking is exacerbated when drilling through formations of depleted or subnormal pressures. Generally the mud weight has been dictated by formation pressures above the zone of depleted pressure, and as a consequence, nothing can be done to offset the large pressure differentials that will exist between the pressure of the mud in the hole and the depleted formation pore pressure. Accordingly, the value P can become very large.
At high borehole angles, the drill pipe may so press against the formation while rotating as to prevent formation of a filter cake and mud lubricity is therefore critically important.
From the early days in the oil patch, oil has been used to loosen stuck drill pipe and to lubricate the borehole before running casing. An oil slug weighted to the same density of the water base mud (a "pill") is placed in and circulated through the mud system in volume adequate to extend from the borehole bottom to a level at least as high as the uppermost point of differential sticking. The oil fluid invades the mud cake, reduces the adhesive forces and lubricates the pipe to reduce friction and facilitate a release of the pipe. Later developed oil emulsion and oil base muds have been favored for preventing differentially stuck drill pipe. Oil emulsion and oil base muds have much lower coefficients of friction than water based muds, and they also lay down very thin filter cakes, which reduces the contact area A.
However, environmental concerns enforced by world-wide governmental regulations increasingly have limited the use of oil emulsion or oil base formulations or oil spotting fluids in drilling operations where the mud might escape into environmentally sensitive areas, such as offshore waters. Recent environmental regulations for offshore drilling fluids require that: (1) no "sheen" be left upon the receiving waters by the drilling fluid; and (2) that the drilling fluid meet stringent toxicity limits as measured by bioassays of Mysidopsis bahia shrimp.
Beyond marine toxicity or other pollution concerns offshore, there is also difficulty and considerable expense in barging and holding large quantities of oil fluids for offshore use; further, all oil muds, and cuttings contaminated with oil muds, must be contained and transported back to shore for disposal. Moreover, land based drilling has the expense of disposing of cuttings and used oil base muds in an environmentally acceptable manner.
In consequence of environmental regulations having the practical effect of precluding use offshore of oil muds, new water base drilling muds have been developed in an effort to obtain performance associated with oil muds. Examples of some modified water base muds are described in U.S. Pat. Nos. 4,830,765, 4,941,981 and 4,963,273 assigned to the assignee of this invention. The muds in these patents include a water soluble component selected from glycol ethers, polypropylene glycols, tripropylene glycol bottoms, polyethylene glycols, ethylene oxidepropylene oxide copolymers, alcohol-initiated ethylene oxide-propylene oxide polymers and mixture thereof.
Water base muds have been described in the patent literature which contain polyoxyalkylene glycol additives to counter the problem of differential drill pipe sticking. U.S. Pat. No. 3,223,622 describes, for use in a spotting pill or as a component of an oil emulsion mud, surfactants which are ethylene oxide adducts of polypropylene glycol in which the molecular weight of the polypropylene glycol is 1500 to 2500 and 20 to 80 weight percent of the molecule is ethylene oxide groups. Stuck drill pipe release compositions involving polyoxyalkylene compounds for spotting pill application are described in U.S. Pat. No. 4,230,587 (polyethylene glycols having molecular weights from 106 to 600, used neat or dissolved in differing proportions in sea water according to molecular weight), and U.S. Pat. No. 4,614,235 (polyalkylene glycol ethers in a weighted water base spotting pill). U.S. Pat. No. 4,466,486 states that polyethylene oxide, ethylene oxide-propylene oxide copolymers, polyvinylmethylether and mixtures thereof in brine can be used in spotting pills to dehydrate and crack filter cake to release stuck pipe. Propoxylated tripropylene glycol bottoms have been used in aqueous spotting fluids.
In spite of developments in modified water base drilling fluids, there remains a need for a water base drilling fluid which provides prevention from sticking of tubular equipment in the borehole of the well due to differential pressure, especially when drilling or when running casing through depleted or subnormal pressure formations. Especially offshore, where high angle holes are drilled from platforms, such a water base drilling fluid is needed that is non-polluting, i.e., environmentally acceptable under currently prevailing regulation requirements mentioned above.